Volume 229, Issue 10

Metastable Ge( cF 136) with empty clathrate-II crystal structure was successfully used for the preparation of otherwise hardly accessible germanium clathrate phases. On reaction of Ge( cF 136) with potassium vapour at 280 °C for 6 days, the new clathrate-II phase K 24 Ge 136 was formed, in which the cages of the germanium framework are completely filled by potassium atoms. The crystal structure of K x Ge 136 is discussed for different potassium contents ( x =0, 8.6, 24).

The isovalent substitution of chromium for aluminium at octahedral site along the Na(Al 1- x Cr x )P 2 O 7 pyrophosphate solid solution has been investigated by means of the combined application of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In agreement with the structural refinements, deconvolution of the optical spectra revealed a progressive decreasing of the crystal field strength parameter 10 Dq moving toward the NaCrP 2 O 7 end-member, meaning that the local chromium–oxygen bond distance increased along the join with the amount of chromium. The calculated structural relaxation coefficient around the substituent ion Cr 3+ was ε =0.97 (i.e., very close to that predicted by the Hard Spheres model). A detailed comparison with compact crystal structures (i.e., perovskite, garnet, spinel, clinopyroxene, and corundum) as well as with other pyrophosphate compounds with II-Na M P 2 O 7 structure-type (with M =Al, Cr, Fe, V, Ti, Mo and In) highlighted that compounds belonging to the Na(Al 1- x Cr x )P 2 O 7 solid solution – characterized by a network connectivity which is basically a framework topology – undergo a structural relaxation that is confined within the first shell (i.e., occurs at the octahedral site), while P 2 O 7 dimers act as rigid units.

Mullite-type (Al 1-x Ga x ) 4 B 2 O 9 compounds were synthesized using the glycerine method. The end members Al 4 B 2 O 9 and Ga 4 B 2 O 9 were produced at different temperatures. Starting from the Al 4 B 2 O 9 composition the incorporation limit of Ga ranges between 60 and 70 mol-%, while starting from Ga 4 B 2 O 9 the Al incorporation limit is 70 mol-% in the Ga 4 B 2 O 9 structure. The crystal structures were refined from the X-ray powder diffraction data and analyzed by the Rietveld method. The positions of B atoms were determined by distance least squares modeling. The temperature-dependent investigations demonstrate that the stability of a given member is a function of Al/Ga ratio in the structure. The incorporation of Ga in the Al 4 B 2 O 9 structures leads to a successive decrease of the decomposition temperatures. Pure Al 4 B 2 O 9 decomposes above 1323 K, whereas pure Ga 4 B 2 O 9 is stable up to 1073 K. The thermal expansions of the lattice parameters were fit using extended Grüneisen first-order approximation for the zero-pressure equation of state. Changes of the internal energy of the crystal were calculated by the Debye-Einstein-Anharmonicity model. The thermal stability of each member has been explained in terms of the obtained Debye temperature.

The N -oxide or 1-hydroxy forms of four solid 1-hydroxy-5-(4-methyl-2-nitroimidazol-2-yl)-4-phenyl-2-(X-phenyl)-imidazole derivatives, 6 , have been studied by X-ray crystallography. Compounds, ( 6 : X=4-Br) and ( 6 : X=2-O 2 N), both recrystallized from EtOH, and ( 6 : X=4-F), recrystallized from EtOCH 2 CH 2 OH, were isolated in the 1-hydroxyimidazole form. In contrast, ( 6 : X=H), recrystallized from EtOCH 2 CH 2 OH, was obtained in the imidazole N -oxide form. Two independent molecules, Mol A and Mol B, rotamers, are present in the asymmetric unit of the imidazole N -oxide compound, ( 6 : X=H) and are linked into chains of alternating compounds, mainly by N–H···O hydrogen bonds, but also by N–H···N and C–H···O hydrogen bonds and π···π interactions. These chains are linked by other C–H···O and C–H···N hydrogen bonds and π···π interactions into a 3-D arrangement. In the 1-hydroxyimidazole compound, ( 6 : X=2-O 2 N), classical hydrogen bonds, O1–H1···N3, are the major interactions linking molecules into C(5) chains: weak π···π stacking interactions also link the molecules within the chains. These chains are linked by C–H···O and hydrogen bonds and by N–O···π interactions into a three- dimensional array. The other 1-hydroxyimidazole compounds, ( 6 : X=4-Br) and ( 6 : X=4-F), have similar structures, which differ from that of ( 6 : X=2-O 2 N). In ( 6 : X=4-Br) and ( 6 : X=4-F), pairs of classical O–H···N hydrogen bonds link molecules into symmetric dimers: these dimers are further stabilized by π···π interactions. Other intermolecular interactions found in both ( 6 : X=4-Br) and ( 6 : X=4-F) are C–H···O hydrogen bond and N–O···π interactions, which link the O–H···N hydrogen bonded dimers into a 3-dimensional array.